The alkylation or transalkylation of benzene with a C2 to C20 olefin alkylating agent or a polyaklyl aromatic hydrocarbon transalkylating agent is one of the primary sources for the production of alkylbenzenes. For example, ethylbenzene is often produced by the alkylation of benzene with ethylene. Ethylbenzene may subsequently be used as a precursor for making styrene by the dehydrogenation of the ethylbenzene. Often, the ethylbenzene and styrene production facilities are integrated in an ethylbenzene-styrene complex so that after the ethylbenzene is produced it is sent to a downstream styrene plant that converts the ethylbenzene into styrene through dehydrogenation. Styrene may in turn be used to produce polystyrene, a widely used plastic, or other products.
In an alkylbenzene production plant, benzene is fed along with a C2 to C20 olefin alkylating agent or polyalkylaromatic hydrocarbon transalkylating agent to an alkylation or transalkylation reactor. Typically, benzene is fed along with ethylene into an alkylation zone, including an alkylation reactor, where alkylation of the benzene and ethylene over an alkylation catalyst forms ethylbenzene. The ethylbenzene product stream typically includes other components as well, such as diethylbenzene. The stream may next be sent to a separation zone where the ethylbenzene is separated from other components in the stream to form a purified ethylbenzene stream.
In an ethylbenzene-styrene complex, the ethylbenzene is next sent to a downstream styrene plant or section of the complex for conversion of the ethylbenzene to styrene. According to one current process, the ethylbenzene is sent to a dehydrogenation reactor within the styrene plant, where a dehydrogenation reaction occurs to form a mixed stream of styrene, benzene, and ethylbenzene. The mixed stream is sent to an ethylbenzene-styrene splitter forming separate ethylbenzene and styrene streams. An inhibitor is typically added to the ethylbenzene-styrene splitter to restrict polymerization of the styrene and corrosion within the splitter. In many instances, the inhibitors include nitrogen compounds. The ethylbenzene stream may be sent to an ethylbenzene recycle column where an ethylbenzene recycle stream is separated from benzene and toluene. The ethylbenzene may be recycled back to the dehydrogenation reactor or reactors in order to produce additional styrene. The benzene and toluene are typically sent to a benzene-toluene splitter where the streams are separated and may be sold.
Catalysts for aromatic conversion processes generally comprise zeolitic molecular sieves. Examples include, zeolite beta (U.S. Pat. No. 4,891,458); zeolite Y, zeolite omega and zeolite beta (U.S. Pat. No. 5,030,786); X, Y, L, B, ZSM-5, MCM-22, MCM-36, MCM-49, MCM-56 and Omega crystal types (U.S. Pat. No. 4,185,040); X, Y, ultrastable Y, L, Omega, and mordenite zeolites (U.S. Pat. No. 4,774,377); and UZM-8 zeolites (U.S. Pat. Nos. 6,756,030 and 7,091,390). It is known in the art that the benzene stream generated by the styrene zone includes contaminants, such as nitrogen, unsaturated aliphatic compounds, and water such that it has been undesirable to recycle the stream back to the alkylation reactor to produce additional ethylbenzene. These contaminants, even at ppm and ppb levels, can cumulatively act to poison the aromatic conversion catalysts, such as aromatic alkylation catalysts and significantly shorten their useful life. More particularly, nitrogen compounds in the benzene stream, as well as water and dienes or other unsaturated aliphatic compounds, are known to deactivate the alkylation or transalkylation catalyst in the alkylation zone and/or transalkylation zone adding additional expense in having to change out or regenerate the catalyst. In addition, due to the contaminants in this stream, the sale of this benzene stream to third parties is generally below the typical market value of benzene
A variety of guard beds having clay, zeolite, or resin adsorbents to remove one or more types of nitrogen compounds and/or other contaminants from an aromatic hydrocarbon stream upstream of an aromatic conversion process are known in the art. Examples include: U.S. Pat. Nos. 7,205,448; 7,744,828; 6,297,417; 5,220,099; WO 00/35836; WO 01/07383; U.S. Pat. Nos. 4,846,962; 6,019,887; and 6,107,535. An acidic molecular sieve H—Y has been utilized to adsorb the nitrogen compounds from the stream.
It was previously identified that unsaturated aliphatic hydrocarbons such as olefinic compounds, and particularly dienes, can shorten the effective life of adsorbents, e.g. nitrogen adsorptive adsorbents, used in the nitrogen removal guard beds that are applied to various process streams, including aromatic hydrocarbon feeds upstream of an aromatic conversion process such as alkylation. These unsaturated aliphatic, e.g. olefinic, compounds are present in aromatic process streams contaminated with nitrogen compounds, including the benzene streams generated in styrene process separation plants and other streams requiring removal of the nitrogen compounds prior to being contacted with a catalyst or other material susceptible to nitrogen poisoning. More particularly, because dienes are typically present at concentrations at least one order of magnitude greater than the concentration of nitrogen compounds in the stream, and compete for adsorbent sites in previous guard beds, they can greatly reduce the capacity of the guard bed. Thus, attempts to simply increase the size of the nitrogen guard bed have been largely ineffective.
Recent attempts, as described in U.S. patent application Ser. Nos. 13/314,796; 13/314,749; and 13/314,842 have focused on removing the olefinic compounds, and in particular dienes such as butadiene or isoprene from a benzene feed stream or a benzene stream exiting the styrene plant prior to directing the stream through a nitrogen compound removal guard bed. In this manner, a large portion of the dienes can be removed from the stream prior to contacting the guard bed catalyst with the stream to restrict the dienes from contacting the guard bed and poisoning the nitrogen adsorbent. With this in mind, these applications propose contacting the benzene or other aromatic stream with adsorbents and/or catalysts including clay, acidic molecular sieves, and/or activated carbon in order to remove at least a portion of the dienes or other C2 to C20 olefin alkylating agent or a poly-alkyl aromatic hydrocarbon transalkylating agents present in a recycle or a fresh feed stream prior to contacting the stream with the nitrogen removal adsorbent to restrict these components from shortening the life of the nitrogen guard bed.
It is also generally known that feed streams to alkylation reactors in an ethylbenzene plant may include water. Particularly, a recycle benzene stream from a styrene monomer production zone has been found to include relatively large amounts of water. Without being bound by theory, it is believed that the H—Y acidic molecular sieve typically used to adsorb the nitrogen compounds from the stream favors water to a much greater extent than nitrogen compounds. In this regard, where water is present in greater amounts than the nitrogen compounds, significant adsorbent capacity is taken up by water, shortening the cycle length or life of the adsorbent. Further, it is believed that certain adsorbents, such as the H—Y acidic molecular sieve used for nitrogen removal may catalyze the polymerization of dienes. This can lead to the blockage of accessible surface area of the adsorbent and coke formation where the adsorbent is regenerated by carbon burn further degrading the effectiveness and useful life of the adsorbent for removing nitrogen components from the stream.
Increases in crude oil prices have created renewed interest in utilizing available streams for recycle in petrochemical processes. Thus, it is desirable to identify ways to utilize feeds and recycle streams in an effective and economical manner for use in aromatic conversion processes while avoiding the problems associated with the presence of the contaminants in the feeds as discussed above.